Wet mill corn processing plants convert corn grain into several different co-products, such as germ (for oil extraction), gluten feed (high fiber animal feed), gluten meal (high protein animal, feed), and starch-based products, including ethanol, high fructose corn syrup, or food and industrial starch. However, because constructing wet-milling plants is complex and capital-intensive, almost all new plants built today are dry mill plants.
Dry milling plants generally convert corn into only two products, i.e., ethanol and distiller's grains with solubles. A typical corn dry mill process consists of four major steps: grain handling and milling, liquefaction and saccharification, fermentation, and co-product recovery. Grain handling and milling is the step in which the corn is brought into the plant and ground to promote better starch to glucose conversion. Liquefaction and saccharification is where the starch is converted into glucose. Fermentation is the process of yeast converting glucose into ethanol. Co-product recovery is the step in which the ethanol and corn by-products are purified and made market ready.
The recovery of ethanol and co-products generally begins with the beer being sent to a distillation system. With distillation, ethanol is typically separated from the rest of the beer through a set of stepwise vaporizations and condensations. The beer less the alcohol extracted through distillation is known as whole stillage, which contains a slurry of the spent grains including corn protein, fiber, oil, and sugars. But these byproducts are too diluted to be of much value at this point and are further processed to provide the distiller's grains with soluble.
In typical processing, when the whole stillage leaves the distillation column, it is generally subjected to a decanter centrifuge to separate insoluble solids or “wet cake”, which includes fiber, from the liquid or “thin stillage”, which includes, e.g., proteins and oil. After separation, the thin stillage moves to evaporators to boil away moisture, leaving a thick syrup that contains the soluble (dissolved) solids. The concentrated syrup is typically mixed with the wet cake, and the mixture may be sold to beef and dairy feedlots as distillers wet grain with solubles (DWGS). Alternatively, the wet cake and concentrated syrup mixture may be dried in a drying process and sold as distillers dried grain with solubles (DDGS). The resulting DDGS generally has a crude protein content of about 29% and is an especially useful feed for cattle and other ruminants due to its by-pass protein content.
While DDGS and DWGS provide a critical secondary revenue stream that offsets a portion of the overall ethanol production cost, it would be beneficial to provide a method and system where a higher protein corn product can be obtained from the whole stillage to be sold at a higher cost per ton than DDGS or DWGS.